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1、www.kaobo100.com中国考博辅导首选学校复旦大学考博英语模拟试题及其解析Flatfish,suchastheflounder,areamongthefewvertebratesthatlackapproximatebilateralsymmetry(symmetryinwhichstructurestotheleftandrightofthebody’smidlinearemirrorimages).Moststrikingamongthemanyasymmetriesevidentinanadultflatfishiseyeplacement:beforematurityone
2、eyemigrates,sothatinanadultflatfishbotheyesareonthesamesideofthehead.WhileinmostspecieswithasymmetriesvirtuallyalladultssharethesameGengduoyuanxiaowanzhengkaoboyingyuzhentijiqijiexiqinglianxiquanguomianfeizixundianhua:silinglingliuliubaliujiuqiba,huojiazixunqq:qiqierliuqibawusanqiasymmetry,memberso
3、fthestarryflounderspeciescanbeeitherleft-eyed(botheyesontheleftsideofhead)orright-eyed.InthewatersbetweentheUnitedStatesandJapan,thestarryflounderpopulationsvaryfromabout50percentleft-eyedofftheUnitedStatesWestCoast,throughabout70percentleft-eyedhalfwaybetweentheUnitedStatesandJapan,tonearly100perc
4、entleft-eyedofftheJapanesecoast.Biologistscallthiskindofgradualvariationoveracertaingeographicrangea“cline”andinterpretclinesasstrongindicationsthatthevariationisadaptive,aresponsetoenvironmentaldifferences.Forthestarryflounderthisinterpretationimpliesthatageometricdifference(betweenfishthataremirr
5、orimagesofoneanother)isadaptive,thatleft-eyednessintheJapanesestarryflounderhasbeenselectedfor,whichprovokesaperplexingquestion:whatistheselectiveadvantageinhavingbothwww.kaobo100.com中国考博辅导首选学校eyesononesideratherthanontheother?Theeasewithwhichafishcanreversetheeffectofthesidednessofitseyeasymmetrys
6、implybyturningaroundhascausedbiologiststostudyinternalanatomy,especiallytheopticnerves,fortheanswer.Inallflatfishtheopticnervescross,sothattherightopticnerveisjoinedtothebrain’sleftsideandviceversa.Thiscrossingintroducesanasymmetry,asoneopticnervemustcrossaboveorbelowtheother.G.H.Parkerreasonedthat
7、if,forexample,aflatfish’slefteyemigratedwhentherightopticnervewasontop,therewouldbeatwistingofnerves,whichmightbemechanicallydisadvantageous.Forstarryflounders,then,theleft-eyedvarietywouldbeselectedagainst
